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A method and apparatus for automated, non-destructive evaluation of spot
welds includes a device for heating a sample containing a spot weld, an
infrared camera for detecting changes in the surface temperature of the
weld, and a computer to acquire and analyze data from the camera. In one
embodiment, the sample is heated on one side and the time-temperature
characteristic is monitored as the heat travels through the sample and
the spot weld. The computer generates a histogram that represents the
relationship between a particular time-temperature characteristic and the
number of pixels exhibiting that characteristic, thereby representing the
quality and size of the weld nugget. By generating a histogram
corresponding to weld quality, the inventive apparatus and method
provides an objective weld quality indicator and allows automation of the
evaluation process.

1. An apparatus for non-destructive evaluation of a weld in a sample,
comprising: a heater that heats a surface of the sample; a heat-detecting
camera that collects temperature data from the sample over a selected
time period; and a computer coupled to the heat-detecting camera to
monitor a time-temperature characteristic from the temperature data and
to compare the time-temperature characteristic with a predetermined
standard indicating weld quality.

2. The apparatus of claim 1, wherein the heater is selected from the group
consisting of a flashlamp, pulsed laser, optical fiber, and light source
coupled with a focusing lens.

3. The apparatus of claim 1, wherein the sample has a first surface and a
second surface, and wherein the heater is disposed opposite the first
surface and the heat-detecting camera is disposed opposite the second
surface.

4. The apparatus of claim 3, wherein the heater heats the first surface of
the sample and the heat-detecting camera monitors the temperature of the
second surface, and wherein the computer measures a time interval between
the initiation of the heating and the time at which at least a portion of
the second surface reaches a predetermined temperature level.

5. The apparatus of claim 4, wherein an image generated by the
heat-detecting camera is made of a plurality of pixels, and wherein the
computer generates a histogram representing the thermal flux through the
weld region.

6. The apparatus of claim 5, wherein the histogram reflects a half-max
time for each pixel in the image, which indicates the time at which the
temperature for the pixel reaches half its maximum temperature.

7. The apparatus of claim 6, wherein the computer evaluates the weld by
comparing at least one of a peak time and a peak amplitude indicated in
the histogram to at least one threshold value.

8. The apparatus of claim 1, further comprising a mirror that reflects
visible light but passes infrared light, wherein the heat-detecting
camera and mirror are disposed opposite a first surface of the sample and
wherein the heater is disposed to reflect the visible light onto the
first surface of the sample.

9. The apparatus of claim 8, wherein the heater heats the first surface of
the sample and the heat-detecting camera monitors the temperature of the
first surface, and wherein the computer measures a time interval between
the initiation of the heating and the time at which at least a portion of
the first surface cools to a predetermined temperature level.

10. The apparatus of claim 9, wherein an image generated by the
heat-detecting camera is made of a plurality of pixels, and wherein the
computer generates a histogram representing the thermal flux through the
weld region.

11. The apparatus of claim 10, wherein the histogram reflects a half-max
time for each pixel in the image, which indicates the time at which the
temperature for the pixel reaches half its maximum temperature.

12. The apparatus of claim 11, wherein the computer evaluates the weld by
comparing at least one of a peak time and a peak amplitude indicated in
the histogram to at least one threshold value.

13. The apparatus of claim 1, wherein an image generated by the
heat-detecting camera is made of a plurality of pixels, and wherein the
computer generates a histogram representing the time-temperature
characteristic for each pixel in the image and evaluates the weld by
comparing the time-temperature characteristic with an objective
threshold.

14. The apparatus of claim 13, wherein the computer evaluates at least one
of a maximum amplitude, ascending slope, descending slope, and maximum
time of the time-temperature characteristic to evaluate the weld.

15. The apparatus of claim 1, further including a conical snout disposed
between said heater and said sample for concentrating the heat generated
by the heater upon the surface of the sample.

16. A method for non-destructive evaluation of a weld in a sample,
comprising the steps of: heating the sample; capturing a plurality of
infrared images of the sample over time; determining a time-temperature
history of the sample; and comparing a time-temperature characteristic in
the time-temperature history with a threshold value to evaluate the weld.

17. The method of claim 16, wherein the heating and capturing steps
include the steps of: heating a first surface of the sample; and
capturing a plurality of infrared images of a second surface of the
sample over time.

18. The method of claim 17, wherein the comparing step includes comparing
at least one time-temperature characteristic selected from the group
consisting of thermal flux through the weld region, a maximum amplitude,
an ascending slope, a descending slope, maximum time value of the
time-temperature history with the threshold.

19. The method of claim 16, wherein the heating and capturing steps
include the steps of: heating a first surface of the sample; and
capturing a plurality of infrared images of the first surface of the
sample over time.

20. The method of claim 19, wherein the comparing step includes comparing
at least one time-temperature selected from the group consisting of a
maximum amplitude, an ascending slope, a descending slope, and maximum
time of the time-temperature history.

21. The method of claim 16, wherein the determining step includes the
steps of: obtaining a time-temperature characteristic for each pixel in
the plurality of images; creating at least one histogram based on all of
the time-temperature characteristics from the obtaining step.

22. The method of claim 21, wherein the time-temperature characteristic is
a half-max time.

23. The method of claim 21, wherein the time-temperature characteristic is
a half-max time, wherein the creating step includes the step of
generating a first histogram indicating a peak time, which corresponds to
the half-max time associated with the greatest number of pixels; and
generating a second histogram indicating a peak amplitude, which
corresponds to a total number of pixels having a half-max time less than
or equal to a selected half-max time.

24. The method of claim 23, wherein the selected half-max time for
determining the peak amplitude is the peak time.

Description

TECHNICAL FIELD

[0001] The invention is directed to weld evaluation, and more particularly
to a method and apparatus for non-destructive evaluation of weld
integrity.

BACKGROUND ART

[0002] Testing weld integrity is an important part of many manufacturing
processes, such as in automotive manufacturing where spot welds are
formed in steel or aluminum sheet metal. Current systems evaluate weld
integrity by prying the weld apart, visually inspecting the weld, and
measuring the diameter of the weld nugget (e.g., the region where the
welding process has formed a true metallurgical bond).

[0003] The problem with known evaluation systems, however, is that the
weld is destroyed during the evaluation process because the weld must be
opened to be visually inspected and measured. Visual inspection also
introduces an element of subjective evaluation, creating the potential
for inconsistent evaluation results between samples. Further, currently
known evaluation methods cannot be automated because they require a great
deal of human intervention and/or user input.

[0004] There is a need for a system that can conduct non-destructive
evaluation of weld integrity and that offers an automation option. There
is also a need for a system and method that can evaluate welds using
objective criteria to ensure consistent evaluation results between
samples.

SUMMARY OF THE INVENTION

[0005] Accordingly, the present invention is directed to an apparatus and
method for non-destructive evaluation of welds. The apparatus generally
includes a heater that heats the weld, a camera that detects changes in
the surface temperature of the weld, and a computer that acquires and
analyzes time-temperature data from the camera as the weld temperature
varies over time.

[0006] The inventive method includes heating one surface of the sample and
starting data collection, in the form of image data, using the camera at
the same time the flash begins. The method then includes examining the
time-temperature history of individual sections in the image obtained
from the camera, such as each pixel in the image. In one embodiment, the
method measures heat flux (i.e. heat per unit time per unit area) during
the time interval between the initiation of the flash and the time a
selected surface of the sample reaches a predefined temperature. At least
one time-temperature characteristic in the time-temperature history is
then compared with objective criteria to evaluate the weld quality.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] FIG. 1 is a schematic drawing of a first embodiment of a weld
evaluation apparatus according to the present invention;

[0008] FIG. 2 is a schematic drawing of a second embodiment of a weld
evaluation apparatus according to the present invention;

[0009] FIG. 3 is an example of a time-temperature history of a center of a
spot weld.

[0010] FIGS. 4A and 4B are examples of histograms that can be used to
evaluate weld quality; and

[0011] FIG. 5 is a flowchart illustrating a method according to the
present invention.

[0012] FIG. 6 is a third embodiment of the present invention using fiber
optic bundles to transmit light energy from the flash lamp to the sample.

[0013] FIG. 7A is an enlarged view of the concentric collar, shown in FIG.
6, used to hold the IR camera and the fiber optic bundles.

[0014] FIG. 7B is a cross section taken substantially along lines 7B-7B of
FIG. 7A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] FIGS. 1 and 2 illustrate two possible configurations of a
non-destructive weld evaluation apparatus according to the present
invention. FIG. 1 illustrates an apparatus 100 having a heat source, such
as a flashlamp 102, disposed adjacent side 104a of a sample 104 having a
weld nugget 106 to be evaluated. Preferably, the same should be capable
of generating an energy density of 2 kilojoules/cm.sup.2 or greater in a
duration of less than 5 milliseconds. Although the description herein
assumes that the heat source is a flash lamp, any device capable of
localized heating of the sample can be used, such as a pulsed laser
optically coupled to an optical fiber, or a light source coupled with a
focusing lens. A heat-sensitive camera, such as an infrared (IR) camera
108, is disposed opposite the other side 104b of the sample 104 in the
embodiment of FIG. 1. The flashlamp 102 is preferably focused to a spot
size that is roughly equal to the weld nugget's 106 diameter. Further, a
conical snout 110 may be mounted on the flashlamp 102 to concentrate the
flashlamp beam onto the sample 104.

[0016] The IR camera 108 is preferably a high-speed focal plane array
camera operating in the 2-5 micron spectral range, although other
spectral ranges can be used. Data is sent from the IR camera 108 to a
computer 112 for capturing the camera data, analyzing the data and either
displaying the results on a display (not shown) or sending the data
electronically to other devices (not shown). The specific manner in which
data is sent to the computer 112 is not significant to the inventive
concept. Although, in a typical system, 12-bit digital data can be
obtained directly from the IR camera 108 using a digital interface PC
card, the same result can be accomplished using analog data. The camera
acquisition rate is preferably high enough to measure the rate of heat
flow through the sample (e.g. 200 Hz or higher).

[0017] To inspect the sample 104 using the configuration shown in FIG. 1,
the flashlamp 102 flash-heats the sample 104 on surface 104a. Data
collection by the IR camera 108 on the opposite surface 104b of the
sample using the IR camera 108 begins at the same time the flash occurs
and continues as the heat travels from one side of the sample 104,
through the weld nugget, toward the other side of the sample 104.
Preferably, data is collected until after surface 104b reaches maximum
temperature (typically this takes a few hundred milliseconds).

[0018] Once the camera 108 and computer 112 collect the data, a software
routine is employed to examine a time-temperature characteristic from the
time-temperature history of each pixel in the image. More particularly, a
dedicated software program operating on computer 112, monitors a time
interval between the initiation of the flash and the time at which the
non-heated surface 104b reaches a maximum temperature.

[0019] FIG. 2 illustrates a second possible configuration for the
inventive apparatus. The apparatus 200 shown in FIG. 2 includes a mirror
202 that reflects infrared light and transmits visible light. As can be
seen in FIG. 2, both the camera 108 and the flashlamp 102 are disposed on
the same side 104a of the sample 104 rather than on opposite sides, as in
FIG. 1. In this case, the visible light generated by the flashlamp 102 is
reflected by the mirror 202 onto the sample's surface 104a to heat the
sample 104, and the heat radiating from the sample 104, in turn, travels
through the mirror 202 to the IR camera to be captured as an image.

[0020] Because the camera 108 and the flashlamp 102 are disposed on the
same side 104a of the sample 104 in the embodiment shown in FIG. 2, the
time-temperature history will be different from that of the previous
embodiment; in this embodiment, the time-temperature history will reflect
an instantaneous temperature rise from the flashlamp heat and then a
gradual cooling. However, the time-temperature histories obtained from
both embodiments can be analyzed in the same manner.

[0021] Referring to FIG. 3, the time-temperature history is used for weld
evaluation in the present invention because the time-temperature
characteristics in the history accurately reflect weld quality. As is
known in the art, in an actual weld joint between two combined and
properly welded pieces of metal the constituent pieces melt and then
resolidify to form a single, continuous weld nugget, rather than two
pieces that are merely in mechanical contact, but with a discontinuous
thermal interface. As a result, the flux of heat (amount of heat per unit
time per unit area) through two properly welded pieces will be greater
than the flux through the constituent pieces joined by some mechanical
means (e.g. pressure, clamping, adhesive, etc.). Furthermore, a weld that
is less than perfect may not have sufficient depth, or may contain
cracks, inclusions or coating residue, porosity due to trapped gas or
other anomalies that will serve to decrease the heat flux through the
weld.

[0022] The expected maximum allowable half-max time (time that the side
opposite the heat source reaches 1/2 of its maximum temperature) for a
weld may be determined empirically or may also be estimated from a known
formula for flash thermal diffusivity measurement, so that 1 t 1 / 2
max = 1.38 L 2 2

[0023] where .alpha. and L are the thermal diffusivity and the thickness
of the sheet metal, respectively. Generating the time-temperature history
and its associated characteristics condenses the entire IR image frame
sequence for a given weld IR into a single, easy-to-analyze data
structure (either an image or the equivalent histogram) that indicates
the quality and size of the weld nugget. Although the weld quality can be
determined from the maximum amplitude, ascending slope, descending slope,
or maximum time in the time-temperature history, a preferred
time-temperature characteristic for evaluating welds is a "half-max"
time, which is the time at which the temperature for a given image
section (e.g. a pixel) rises to half its maximum value. The half-max time
is preferred because it can be measured precisely without extensive
signal processing, is independent of signal amplitude, and is not
adversely affected by noise. The specific manner in which a
time-temperature history and characteristic is analyzed will be explained
in greater detail below.

[0024] FIG. 3 illustrates one example of a time-temperature history for a
point in the center of the nugget of two spot welds, one good and one
bad, for explanatory purposes. The welds used in this example were formed
from identical thicknesses of sheet steel and destructively evaluated via
conventional visual inspection. As can be seen in the Figure, the maximum
amplitude, ascending and descending slopes, and maximum time values
differ for the two welds due primarily to the heat flow interruptions and
resulting increased thermal transit time in the weld. Although any of
these characteristics could be used to form an image, the half-max time
is the preferred for weld evaluation, as noted above.

[0025] After measuring the time-temperature characteristic, such as the
half-max time, for each pixel in the image, the computer may generate a
map of the pixel half-max times as new image. This image provides a
visual indicator of the weld's quality because the shortest half-max
times are expected at the weld nugget if the weld is properly
constructed. If the half-max times in the welded area are longer than
allowed for a particular alloy and sheet metal thickness, the weld is
rejected because the longer half-max times indicate thermal transit
delays caused by imperfections in the weld. Similarly, if the number of
pixels with acceptably short times is less than the minimum area allowed
by a predetermined standard for weld nugget size, the weld is rejected
because the small number of acceptable pixels indicates that the weld
nugget area is unacceptably small.

[0026] Referring to FIGS. 4A and 4B, the image of the half-max time
described above is useful for visualizing the weld quality, but does not
easily lend itself to an automated process. Evaluating the weld quality
using an integral of the half-max time image as well as the original
half-max time image, as illustrated in FIGS. 4B and 4A, respectively,
provides the capability for automated weld evaluation. In FIG. 4A, the
half-max time for each pixel in the image obtained from the IR camera is
used to generate a two-dimensional array, or a histogram, that documents
the half-max times appearing in the image and the number of pixels
exhibiting each half-max time. The number of pixels that occur within a
given time interval of this histogram represent the thermal flux through
the weld region. The histogram of a good weld will indicate a flux value
greater than some threshold value during the time interval that begins at
the onset time, i.e. the time at which the first arrival of heat
transmitted through the sample occurs. Further, the number of pixels can
be directly correlated to area by performing a one time calibration
process consisting of placing a ruler or other object of known length and
width in the IR camera's field of view and dividing the linear dimension
by the number of pixels spanned. Thus, the half-max time histogram can
provide a great deal of information about the physical characteristics of
the weld without requiring any user to observe the image generated by the
IR camera.

[0027] Once the half-max time histogram has been created, weld acceptance
and rejection decisions can be made on the basis of the total number of
pixels having a half-max time that falls below a chosen threshold time.
It is important to note that we are measuring the thermal flux, as it
reaches the back surface, and not the mere absolute velocity of
temperature change. This is an important consideration, because in actual
automated welding operations, the applied pressure of the weld gun may
compress the molten material so that thickness variations are introduced
as high as 30% of the nominal thickness. Although thickness variations
will affect the transit time of heat through the weld, it will not affect
the flux of heat through the weld. In order to compare separate welds
that may have such thickness variations, a compensation adjustment can be
made to the onset time of all welds to match that of a selected
calibration standard.

[0028] FIG. 4A shows the histograms for a good weld 300 and a bad weld
302. The peak in each histogram 300, 302 indicates the half-max time for
the greatest number of pixels in the image. As can be seen in the Figure,
the good weld histogram 300 has a peak around 30 milliseconds, while the
bad weld histogram 302 has a peak around 40 milliseconds, indicating that
a large number of pixels in the bad weld have unacceptably long half-max
times.

[0029] For automation purposes, the flux as measured by the integral of
the half-time histogram is particularly useful. FIG. 4B illustrates the
calculation of the flux using the integral of the half-max time histogram
shown in FIG. 4A, again for a good weld 304 and a bad weld 306. As can be
seen in FIG. 4B, the integrated histogram for the good weld 304 indicates
that at 35 milliseconds after the heat pulse, approximately 2000 pixels
on the good weld have half-max times shorter than 35 milliseconds, while
the integrated histogram for the bad weld 306 indicates that no pixels
have half-max times of 35 milliseconds or less. From these histograms, it
is possible to evaluate whether the weld is good without having to
actually view the image obtained from the IR camera, making it possible
to automate the weld evaluation process.

[0030] FIG. 5 is a flowchart that describes one embodiment of the
inventive method. As noted above, the method 500 begins with the computer
initiating the weld inspection process at step 502. The welded sample is
flash-heated at step 504, and image data capture begins at the same time
as the flash heating step at step 506. Next, high-speed IR image data is
captured and stored at step 508. The captured image data is used to
calculate the half-max time or other time-temperature characteristic for
each pixel in the image at step 510. The half-max time information is
then used to generate at least one histogram, as explained above and as
shown in FIGS. 4A and 4B, at step 512. The histogram is analyzed to find
a peak time (i.e., the half-max time corresponding to the greatest number
of pixels) and a peak histogram value, or a peak amplitude (i.e., the
total number of pixels having a half-max time less than or equal to a
particular half-max time, such as the peak time) at steps 514 and 516,
respectively. This analysis can be conducted by integrating the half-max
time histogram to obtain a second histogram, such as the one shown in
FIG. 4B, but is not limited to such a method.

[0031] Once all of the peak time and peak amplitude information is
obtained for the image, the method 500 determines whether the peak time
exceeds a threshold value at step 518. Exceeding this threshold indicates
a bad weld (step 520) because the heat flow is apparently slowed due to
flaws in the weld. The method 500 also evaluates whether the peak
amplitude (e.g., number of pixels) at the peak time, which corresponds to
an area amount on the weld, is greater than a minimum allowable area at
step 522. Falling below the minimum allowable area indicates a bad weld
(step 524), even if the peak time falls below the threshold time in step
518, because it indicates that the weld nugget is not large enough to
establish an acceptable bond. In other words, even if the largest number
of pixels have a half-max time lower than a selected time threshold, the
total number of pixels at the peak time may not be enough to create an
acceptable weld. If the histogram generates acceptable results at steps
518 and 522, the weld is judged to be a good weld at step 526.

[0032] As can be seen from the above description, the method shown in FIG.
5 relies on objective criteria and does not require any human
intervention or input. Thus, the process can be completely automated.
Further, the objective criteria used in the invention provides an
unambiguous way to set a weld quality threshold based on thermal transit
time and the welded area size, removing the chance of inconsistent
evaluations caused by subjective visual assessments. Because the weld
quality can be characterized by the invention using two parameters, peak
amplitude (weld nugget area) and peak time, the weld quality data can be
archived and compared to other welds for process control. This capability
does not exist for any known weld evaluation apparatus or method.

[0033] Now referring to FIG. 6, in a third embodiment, the present
invention includes dual flashlamps 602, 604 which are electrically
powered by their respectively associated power supplies 606, 608. Heat
energy from flashlamp 602, 604 is delivered to sample 104 by way of a
respectively associated optical fiber bundle 610, 612. Although the use
of two flashlamps 602, 604 is the preferred embodiment, the current
arrangement can be used with a single flashlamp or three or more
flashlamps. In order to increase the transfer efficiency between the
flashlamps and their respectively associated optical fiber bundles, a
respectively associated concentrator 614, 618 is disposed between each
flashlamp 602, 604 and its respectively associated optical fiber bundle
610, 612. The concentrator can be any number of well-known means for
concentrating light such as a lens or a tapered snout reflector attached
to each fiber bundle. In a preferred embodiment, each fiber bundle
terminates adjacent a respectively associated lens 620, 622 (see FIG.
7B). Lenses 620, 622 reduce light beam divergence.

[0034] Because optical fibers are inherently lossy, it may be necessary to
use several flashlamps in order to deliver sufficient heat energy to the
sample. Notwithstanding this inherent disadvantage in using optical
fibers, there are a) fiber optic bundles are small (each bundle is on the
order of 0.25 to 0.5 inches in diameter) and can be placed close to the
sample surface, and b) because the heat source can be located far away
from the exit end of the fiber bundle, the exit aperture of the fiber
bundle does not experience any significant rise in temperature and
accordingly does not introduce artifacts into the infrared image. Also,
the flexibility of the fiber bundle is advantageous in some manufacturing
situations, specifically where the imaging camera 624 and associated
collar 626 may be mounted on a robot or actuator. One preferred
configuration for this embodiment would be to house the exit ends of four
fiber bundles in a respectively associated aperture 628 through 634 of
collar 626 such that the exit ends of the bundles were aimed at the weld
and at a prescribed distance from the weld. Collar 626 would be mounted
concentrically to the lens of infrared camera 624 such that the camera
lens and the convergence point of the light beams emanating from the
fiber bundles coincide. FIGS. 7A and 7B disclose a preferred embodiment
for collar 626. In this preferred embodiment aperture 636 is formed in
the center of a generally cylindrical body. Four apertures 628, 630, 632,
and 634 are spaced equally around aperture 636 and form the means for
carrying a respectively associated optical fiber bundle. Preferably each
aperture 628, 630, 632, and 634 is terminated by a lens (see for example
lenses 620, and 622 covering apertures 628, and 630 respectively) for
reducing light beam divergence. Collar 626 can be constructed from any
material which is capable of withstanding the mechanical and heat loads
associated with the environment in which the present invention is used.
It is anticipated that materials such as aluminum, steel, plastic,
reinforced resins, or the like will be preferred materials in the
construction of collar 626.

[0035] The present invention is therefore a method and apparatus that
tests weld quality without destroying the weld during testing. The
invention can determine both the size of the weld and the quality of the
bond via the weld's time-temperature characteristics allowing the
invention to evaluate weld quality with no human intervention.

[0036] It should be understood that various alternatives to the
embodiments of the invention described herein may be employed in
practicing the invention. For example the present invention has been
discussed in the context of examining weld integrity; however, it is
contemplated that the present invention can be used in any application
where the quality of the bond between two joined materials is at issue.
Such applications lo could include examining adhesively joined laminated
materials or materials whereby a base member is coated or painted by one
or more deposited materials. It is intended that the following claims
define the scope of the invention and that the method and apparatus
within the scope of these claims and their equivalents be covered
thereby.